1. Field of the Invention
This invention relates to the concentration of phosphate minerals from their ores and particularly relates to the beneficiation of phosphate ores by flotation of the silica particles therein.
2. Review of the Prior Art
In the Florida phosphate industry, the grades of calcium phosphate having as much as 35-40% bone phosphate of lime (BPL), Ca.sub.3 (PO.sub.4).sub.2, are presently decreasing, particularly because ores containing coarse phosphate pebbles are diminishing in quantity and being replaced with ores containing finer phosphate particles having about the same size range as coarse sand.
About forty years ago, concentration methods were developed and extensively practiced for beneficiation of phosphate minerals in Florida and elsewhere by means of negative-ion or anionic agents for preferentially collecting the phosphate values in the ore by flotation thereof, but a considerable percentage of these values often went to waste with the tailings. In 1942, the Crago patent, U.S. Pat. No. 2,293,640, disclosed the use of a negative-ion agent to collect and remove from an ore a rougher concentrate containing a high proportion of the phosphate values mixed with some siliceous gangue and thereafter removing the negative-ion agent from this rougher concentrate and treating it with a positive-ion agent to collect and remove therefrom by froth flotation most of the siliceous gangue contained therein.
U.S. Pat. No. 2,313,360 shortly thereafter disclosed a method for preferentially floating the siliceous gangue from phosphate ores with primary aliphatic amines, such as octadecylamines at 1.25 pounds per ton of ore. This flotation, however, had to be conducted on the alkaline side, such as at pH values of about 8.5 to 10 or 11. Preferably, the ground ore was conditioned by contact with an alkaline solution for a few minutes, then washed with water to remove adhering alkali, and formed into a slurry having the desired pH.
U.S. Pat. No. 2,750,036 teaches a process for anionic conditioning with reagents including NaOH, fatty acids, and fuel oil, retarded flotation to remove a high grade froth phosphate product, again conditioning the underflow with anionic reagents, and scavenger flotation to produce a froth concentrate which is mixed with sulfuric acid, rinsed, and floated with positive-ion reagents to produce a siliceous froth which is sent to waste and a PO.sub.4 concentrate that is added to the first concentrate product.
U.S. Pat. No. 3,013,664 described a process for flotation with a cationic flotation agent of a raw phosphate rock feed having a particle size of about -14/+150 mesh, desliming through a desliming cyclone, flotation with a cationic flotation reagent of a mixture of raw feed and a recirculated material to remove overflow or float comprising fine silica and activated coarse silica, and then conditioning the underflow or first rougher concentrate with an anionic flotation reagent and floating to remove the phosphate values as the overflow.
U.S. Pat. No. 3,388,793 is directed to washing and sizing a phosphate matrix to remove +16 mesh pebble and to deslime the -16 mesh fraction by removing the -150 mesh slimes, next to screen the -16/+150 mesh material to separate it into -16/+35 mesh material and -35/+150 mesh material, both of which are subjected to conditioning with an anionic reagent and rougher flotation to produce a combined concentrate which is acid scrubbed and then floated with a cationic reagent to produce a sink product consisting of phosphatic materials as the final concentrate.
U.S. Pat. No. 3,099,620 teaches the flotation of an unsized ore with an anionic reagent, dewatering both the froth and the underflow, mixing both with sulfuric acid and washing, and floating with a cationic reagent to produce phosphate containing underflow concentrates and overflow discard tails.
U.S. Pat. No. 3,349,903 describes a complicated process for maintaining desired solids concentrations of a -5 mesh feed controlling pH to the range of 8.5-11 to produce a first rougher feed which is floated with an anionic flotation reagent to produce a phosphate rich rougher overflow concentrate and a phosphate poor rougher tailing. The latter is deslimed, dewatered, mixed with water in two stages to 20-30% solids, projected to a pH of 6.8-7.3, and scavenger floated with a cationic flotation agent to produce a phosphate rich scavenger underflow tailing which is recovered. The phosphate rich overflow concentrate is treated with mineral acid, partially deoiled and dewatered, reconstituted with water to 20-30% solids, adjusted to a pH of 6.8-7.3, treated with a cationic flotation agent and subjected to cleaner flotation to produce phosphate rich cleaner underflow concentrate.
U.S. Pat. No. 3,388,793 describes a process for treating a deslimed phosphate ore by screening on a 35 mesh screen, adding an anionic reagent with coarse and fine fractions to obtain a phosphate-rich froth which is combined, scrubbed with acid, treated with a cationic reagent, and floated to obtain a tail as a product.
In general, present flotation practice for concentrating Florida phosphate ores is to use a two-stage process involving: A. conditioning the -14/+150 mesh ore at 60-70% solids with fatty acids at about 1 lb/ton, fuel oil at about 2 lbs/ton, and NH.sub.3 or NaOH to produce a pH of 8-10; B. rougher flotation of phosphates (froth or rougher concentrates) from coarse silica (rougher tailings) to produce a rougher concentrate having 50-60% BPL; C. deoiling of the rougher concentrates with concentrated sulfuric acid; D. conditioning the rougher concentrates at 60-70% solids with fuel oil at 0.5 lb/ton and a C.sub.16 -C.sub.18 primary amine acetate at 2 lbs/ton; and then E. cleaner flotation of the fine silica from the rougher concentrates to remove fine silica from the phosphate product having less than 5% insolubles and 70-72% BPL. Present phosphate recoveries are 70-80%.
This two-stage reverse flotation process, requiring removal of the anionic reagent with acid and, in some cases, pH control, is troublesome, expensive, and increasingly inefficient as the industry moves southward from Polk County to Hardee County in Florida and the available ores become increasingly lower in grade.
Other factors that are important in the reverse flotation method that is current used are that well water or limestone water must be used and depressants and/or deflocculators are often needed.
Accordingly, a simpler process is needed at the present time. This process should require fewer controls, fewer process steps, non-critically as to pH, non-critically as to water quality, and no need for depressants and deflocculators. It should also have the capability to operate on the lower grade ores that are increasingly necessary to use and be more efficient in phosphate recovery on such ores.